There is a need to perform communications or sensing by attaching a wireless node operable to communicate or sense using a radio wave, on a wall or a ceiling of an inside space of a goods container, a storehouse, an office, or the like. For example, such a need arises from the purpose of easily constructing a local area network using a wireless system in the office, or from the purpose of monitoring opening and closing of a door of the goods container or presence of a hole drilled through the wall from the inside.
http://www2.crl.go.jp/kk/e412/CRL_News/back_number/222/222.htm As mentioned in the description on the above WEB site (Japan Ministry of Posts and Telecommunications Communications Research Laboratory: CRL NEWS 1994.9 No.222 “To achieve high-speed wireless LAN in offices”), when using a radio wave having a millimeter wavelength band for high-speed transmission in a given space, there occurs a transmission distortion due to multipath propagation caused by reflections from a wall, a ceiling, a floor, a utensil, and the like. In addition, there occurs an interruption of a propagation path due to presence of objects like the utensil in the space, or persons walking there. These phenomena lead to significant problems in the system. Generally, when a radio wave of a circularly polarized wave is launched into a wall at a small incident angle and is then reflected off the wall, the polarized wave will reverse its direction of rotation. In a known system, the use of the circularly polarized wave upon the transmission and reception permits excessive reduction in received multipath waves that are caused by an odd number of the reflections of the polarized wave from the wall.
However, in a metal box such as the goods container, there occurs a great number of the multipath reflections. (It is noted that an object whose inner surface is made of metal, and an object serving as a conductor in view of electromagnetics, even if it is composed of the metal with another thin painting or coating, are also hereinafter referred to as the “metal box”.) For example, by using a wireless node for transmitting and receiving a right-handed circularly polarized wave in the goods containers, it is possible to reduce considerable influences of the multipath propagation, but harmful influences thereof possibly remain upon communication between the wireless nodes or measuring of a distance between them in the goods container.